Various iron materials have been used to improve methane production rate in the anaerobic sludge. The effect of iron materials including zerovalent iron (ZVI), magnetite (Fe3O4) and iron (III) oxide (Fe2O3) on the methane production rate was compared. It was found that all the iron materials can improve the methane production rate, and ZVI, Fe3O4 and Fe2O3 increased by 19.8%, 13.7% and 17.9% in which the ZVI performed better on improving the methane production. Then the long-term effects of ZVI on the methane production rate by anaerobic organisms was further studied. ZVI increased the methane production rate by 7.2%, the improvement was unstable and decreased due to the corrosion and loss of the ZVI in the anaerobic wastewater treatment system. The improved performance was recovered after the ZVI was re-added into the anaerobic sludge, the methane production rate increased by 6.1%. The added ZVI obviously enriched exoelectrogenic archaea Methanobacterium due to its ability of direct interspecific electron transfer, and its relative abundance was up to 61.99%. The enhanced direct interspecific electron transfer was disclosed as the main mechanism involved in enhanced methanogenesis in the anaerobic sludge.